A pacemaker is a medical device for implant within a patient that recognizes various arrhythmias such as tachycardia and delivers pacing therapy to the heart in an effort to remedy the arrhythmia. An ICD is a device, also for implant within a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate the fibrillation. Within pacemakers and ICDs, it is important to distinguish a tachycardia that arises in the ventricles from those that arise elsewhere in the heart. A tachycardia that arises in the ventricles (referred to as VT) is often more serious than a tachycardia arising elsewhere in the heart, since VT can sometimes lead to VF, which is fatal if untreated. Moreover, the type of therapy to be delivered to the heart of the patient depends upon the source of the tachycardia. In particular, it is important to discriminate SVT from VT. SVT is a tachyarrhythmia whose origin is above the ventricles but which is conducted into the ventricles, resulting in unacceptably rapid ventricular rate. The true underlying arrhythmia in these cases may be AF, sinus tachycardia (ST), ectopic atrial tachycardia, atrial reentry tachycardia, atrioventricular (NV) nodal reentry tachycardia, paroxysmal AF or atrial flutter.
Failure to distinguish SVT from VT can result in delivery of inappropriate therapy. Depending upon the capabilities of the implanted device, inappropriate therapy might include delivery of unnecessary anti-tachycardia pacing (ATP) to the ventricles in response to an SVT (that has been misidentified as VT) or delivery of unneeded cardioversion shocks to the atria in response to a VT (that has been misidentified as SVT.) Misidentification of SVT and VT is one of the leading causes of improper device therapy, inclining the delivery of painful and unnecessary cardioversion shocks.
Accordingly, it would be highly desirable to efficiently and reliably distinguish SVT from VT. Waveform morphology comparison may be employed in an attempt to achieve such discrimination. Waveform discrimination techniques are described in, e.g., U.S. Pat. No. 5,273,049 to Steinhaus, et al. entitled, “Detection of Cardiac Arrhythmias using Template Matching by Signature Analysis”; U.S. Pat. No. 5,240,009 to Williams, entitled “Medical Device with Morphology Discrimination”; U.S. Pat. No. 5,779,645 to Olson, et al., “System and Method for Waveform Morphology Comparison”; and U.S. Pat. No. 6,516,219 to Street, entitled “Arrhythmia Forecasting based on Morphology Changes in Intracardiac Electrograms.” See, also, the improved morphology-based discrimination techniques described in U.S. patent application Ser. No. 11/674,974, filed Feb. 14, 2007, of Graumann, entitled “System and Method for Morphology-Based Arrhythmia Discrimination using Left Ventricular Signals sensed by an Implantable Medical Device.”
Other techniques for distinguishing VT and SVT include “sudden onset discrimination” and “PR logic discrimination.” Sudden onset discrimination is discussed, e.g., in U.S. Pat. No. 6,636,764 to Fain, et al., entitled “Safety Backup in Arrhythmia Discrimination Algorithm.” PR logic discrimination is discussed, e.g., in U.S. Pat. No. 7,058,450 to Struble, et al., entitled “Organizing Data according to Cardiac Rhythm Type.”
However, waveform discrimination, sudden onset discrimination and PR logic discrimination may not be optimal or sufficient in distinguishing VT from SVT, and inappropriate cardioversion shocks continue to be a problem. Also, accurate waveform discrimination can be computationally intensive, thereby consuming device resources.
Accordingly, it would be desirable to provide improved techniques for distinguishing VT and SVT and aspects of the invention are directed to that end.